The present invention relates to noise suppression in radio receivers and more particularly to noise blanking circuits with squelch.
Blanking circuits for use in communication radio receivers are well known in the art. Generally, the blanker circuits detect noise signals introduced by the transmitting medium and which are received at the antenna along with transmitted information signals. Several means for detecting the noise have been devised. The detected noise signals are processed whereby a blanking signal is produced which blanks the receiver for the duration of a blanking signal, thus preventing a noise burst from being heard by the listener.
It has been proposed heretofore, to provide in a radio receiver, an RF amplifier stage receiving transmitted radio frequency signals and noise signals from a receiver antenna, the output of the RF amplifier stage being supplied to one input of a mixer stage having a second input supplied with a signal from a local oscillator, the mixer converting the radio frequency signal from the amplifier to an intermediate frequency and supplying the intermediate frequency signal to one input of a blanker gate which receives a blanking signal from a blanking signal source at a second input thereof. The blanker gate in normal operation, passed intermediate frequency signals from the mixer stage to an intermediate frequency filter and amplifier stage, but in response to a blanking signal from the blanking signal source, the blanker gate decouples the mixer from the intermediate frequency filter and amplifier stage. Signals from the IF filter and amplifier stage are detected and demodulated by means of an amplitude limiter and discriminator, into audio signals, and these audio signals are applied via an audio amplifier to a transducer such as a loud speaker.
The audio signal output from the amplitude limiter and discriminator, is applied to a noise squelch circuit which detects noise frequencies above a predetermined frequency. If the noise frequencies exceed a selected amplitude, a squelch signal is generated and supplied from the squelch circuit to the audio amplifier to block or mute the audio signals.
Current FM squelch circuits use peak detectors and no noise blanking, and are susceptible to spiky noise.